Flood-aided hot fluid soak method for producing hydrocarbons

ABSTRACT

Relatively viscous liquid hydrocarbons are produced from a subterranean hydrocarbon reservoir by opening at least one well borehole into fluid communication with the reservoir at least at two spaced locations. A relatively mobile hot fluid, such as steam, is injected through the well borehole and into the earth formation around one of the locations for a period of time sufficient to form a hot zone and thermally mobilize liquid hydrocarbons present in the earth formation. Formation fluids are produced, by backflowing fluid from the hot zone, while a hydrocarbon-displacing fluid is being injected into the earth formation around another of the locations within the reservoir to displace hydrocarbons into the hot zone.

1 w SR Unite States Patent Prats [54] FLOOD-AIDED HOT FLUID SOAK METHOD FOR PRODUCING HYDROCARBONS [72] inventor: Michael Prats, Houston, Tex. [73] Assignee: Shell Oil Company, New York,N.Y. 221 Filed: Dec. 16, 1910 [21] Appl. No.: 98,938

Related U.S. Application Data [63] Continuation-in-part oi- Ser. No. 827,750, May 26,

1 Y OlL DISPLACING GAS FLUID IN GAS SEPARATOR PUMP SEPARATOR 3,245,467 4/1966 Fitch ..166/273 3,185,634 5/1965 Craig, Jr. et al. ..166/273 3,254,714 6/1966 Gogarty et a1 ..166/274 3,259,186 7/1966 Dietz ..166/263 3,358,759 12/1967 Parker... .....l66/263 3,379,246 4/1968 Sklar et .166/251 3,380,527 4/1968 Craighead ..166/272 Primary Examiner-Stephen J. Novosad Attorney-H. W. Coryell and Harold L. Denkler [5 7 ABSTRACT Relatively viscous liquid hydrocarbons are produced from a subterranean hydrocarbon reservoir by opening at least one well borehole into fluid communication with the reservoir atleast at two spaced locations. A relatively mobile hot fluid, such as steam, is injected through the well borehole and into the earth formation around one of the locations for a period of time sufficient to form a hot zone and thermally mobilize liquid hydrocarbons present in the earth formation.

Formation fluids are produced, by backflowing fluid from the hot zone, while a hydrocarbon-displacing fluid is being injected into the earth formation around another of the locations within the reservoir to displace hydrocarbons into the hot zone.

11 Claims, 5 Drawing Figures OIL OIL l 1 HEAT HEATER HEAT EXCHANGER EXCHANGER N STEAM 22 IN l omega o v 0 ,ofif gg 1 3 0 0 n i o PKTENTED DEC 26 I972 SHEET 1 [IF 2 IOd SEPARATOR OIL DISPLACING HEAT EXCHANGER U) [N FIG.

GAS

OIL DISPLACING FLUID IN GAS SEPARATOR PUMP HEATERI SEPARATOR OIL HEAT

EXCHANGER STEAM STEAM l N OIL HEAT EXCHANGER FIG 2 INVENTOR:

. MICHAEL PRATS HIS ATTORNEY PATENTED B15826 I972 SHEET 2 BF 2 VISCOSITY OF DRIVE FLUID TEMPERATURE DISTANCE FROM WELL BORE HOLE 22 DISTANCE FROM WELL BORE HOLE 23 FIG. 3

FIG.

INVENTOR'.

MICHAEL PRATS BY f HIS ATTORNEY FLOOD-AIDED HOT FLUID SOAK METHOD FOR PRODUCING HYDROCARBONS CROSS-REFERENCE This application is a continuation-in-part of copend- 'ing U.S. Pat. application, Ser. No. 827,750, filed May 26, 1969, now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to an improved method for recovering relatively viscous liquid hydrocarbons from subterranean earth formation. More particularly, the invention is directed to a thermal recovery process wherein a hydrocarbon-displacing fluid is injected into one location within the formation while a hot fluid injection-backflow operation, such as a steam-soak operation, is being carried out at another location in the same earth formation.

The term thermal soak is used to refer to a recovery process in which a hydrocarbon-containing formation is heated by injecting and then back-flowing a relatively mobile hot fluid, such as steam, into and out of the formation. After a predetermined quantity of hot fluid has been injected, the well is normally shut in for a soaking period. The length of the soaking period is adjusted so that a substantial quantity of the latent heat of the inflowing fluid is transferred to the formation to heat the hydrocarbons contained therein to reduce their viscosity and make them more mobile. After the soaking period, the formation hydrocarbons are produced from the heated zone by making substantially any type of reservoir drainage or other producing mechanism, as for example gravity drainage, or solution gas drive, and removed from the well borehole by pumping or other production methods.

Where steam is used as the injected hot fluid, the steam may be high-quality, substantially dry steam or may be low-quality steam containing a considerable amount of water in a liquid phase. A steam soak process using a low-quality steam is more particularly described and claimed in U.S. Pat. No. 3,193,009. The steam may be injected into the hydrocarbon-containing formation for a few days or weeks. The length of time that the steam is injected is determined by the size to which the heated zone is to be extended, by the viscosity of the oil and the permeability and other formation characteristics that affect the efficient transfer of the latent heat of the steam to the hydrocarbons contained in the formation. After the soaking period, the well borehole is placed back on production using conventional producing mechanisms normally of the depletion type of drive in combination with pumping or similar lift methods.

Typically, thermal drive oil recovery processes such as steam drives use spaced injection and production wells completed into an underground earth formation containing viscous hydrocarbons that are to be produced. In such processes, steam is injected through one of the wells while fluid is produced through another of the wells. The temperature of the steam reduces the viscosity of the hydrocarbons and the pressure and flow of the steam drives the heated hydrocarbons towards the producing well. However, during the initial injection, the hydrocarbons around the production well and in other areas remote from the injection well remain at original formation temperature and,

therefore, remain relatively immobile. As a consequence, when using economically feasible injection pressures, increased production of viscous hydrocarbons is not significant during the early stages of a thermal drive process. A significant increase in the production is notattained until sufficient hot fluid has been injected into the injection well to heat substantially all of the portion of the formation that is located between the injection and production wells.

Steam soak process, as for example those described in U.S. Pat. Nos. 3,259,186 and 3,292,702, are generally effective in producing hydrocarbons from viscous hydrocarbon reservoirs that are relatively thick and generally uniformly permeable. However, since steam is lighter than the oil and water which is normally present in the pores of such a reservoir, the injected steam tends to rise. This effect, known as gravity layover or gravity segregation, causes the steam zone to expand radially along the top of the reservoir to a much greater extent than it does along the bottom of the reservoir.

It is well known in the oil production art to inject an oil-displacing fluid into an oil reservoir to move the oil present in the reservoir toward a well borehole or a zone from which fluid is to be or is produced. This oil production operation is commonly referred to as a flooding or displacing process. Such a process generally uses an inexpensive oil-displacing fluid such as water or an aqueous solution, which may contain a viscosity enhancer of the type described in U.S. Pat. Nos. 2,827,964 and 3,039,529. Alternatively, such process may use soluble oils of the type described in U.S. Pat. No. 3,254,714. Where the viscosity of the oil present in the formation is relatively high, the oil-displacing fluid must have a comparable viscosity in order to avoid bypassing much of the oil. The rates at which relatively viscous fluids can be moved through hydrocarbon-bearing reservoirs, without applying a pressure that fractures the reservoir rock, are often too low for a commercially feasible oil production process.

SUMMARY OF THE INVENTION It is an object of this invention to provide an improved method of producing liquid hydrocarbons from a hydrocarbon-bearing subterranean earth formation by combining a hot fluid soak process with a viscous oil displacement process while avoiding the disadvantages inherent in the individual processes.

It is a further object of this invention to provide such a method which produces hydrocarbons at a relatively rapid rate during the backflow cycle of a hot fluid soak operation.

These and other objects are preferably accomplished by opening at least one well borehole into fluid communication with a hydrocarbon-bearing subterranean earth formation at least at two spaced locations (preferably separated vertically) in the formation. A relatively mobile hot fluid is injected into the earth formation around one of the locations for a period of time sufficient to thermally mobilize fluids present in the earth formation and form a hot zone. Formation fluids are produced from the hot zone while a substantially non-bypassing hydrocarbon-displacing fluid is injected into the earth formation around another of the locations within the earth formation to displace hydrocarbons into the hot zone. Hydrocarbons are recovered from the fluid produced from the hot zone.

The relatively mobile hot fluid that is injected can comprise substantially any fluid that is or becomes signiflcantly hotter and more mobile than the reservoir hydrocarbon at the normal reservoir temperature. Such a fluid is injected into the reservoir so that it moves around or bypasses, at least some of the reservoir hydrocarbon, transfers heat to the bypassed hydrocarbon, and subsequently, entrains and/or displaces the heated hydrocarbon when fluid is flowed back into the well during a backflow or production cycle of a hot fluid soak oil recovery process. Such a fluid can comprise steam or hot water or substantially any hot mobile fluid that is not extensively miscible with the reservoir hydrocarbon. Such a fluid can be heated at a surface or downhole location in or near the borehole of a well; for

example, steam or hot water can be generated or heated in a surface-located boiler or heater or in a downhole boiler or heater or by means of an in situ combustion within the reservoir; or, a mobile reactive fluid such as an oil reactant of the type described in US. Pat. No. 3,250,328 can be injected at ambient temperature and heated chemically within the reservoir; or the like.

The substantially non-bypassing hydrocarbon-displacing fluid that is injected may comprise substantially any fluid that: (l) is sufficiently mobil to flow into the hydrocarbon-bearing subterranean earth formation at a significant rate (e.g., at at least about 100 barrels per day), in response to an injection pressure of less than the fracturing pressure of the earth formation; and (2) has a mobility at least substantially as low as that of the hydrocarbon-containing fluid in the earth formation near the location at which the hydrocarbon-displacing fluid is injected. For example, such a hydrocarbon-displacing fluid may comprise (a) a foam or relatively low mobility mixture of gas and liquid, (b) a relatively low mobility thickened aqueous liquid (i.e., an aqueous solution or dispersion of a thickening agent such as a partially hydrolyzed polyacrylamide), (c) an oil solvent that dissolves sufficient earth formation hydrocarbon to form a solution having a frontal portion mobility substantially as low as that of the reservoir fluid, (d) a combination of a chemical slug such as an aqueous or oil external dispersion or micellar solution of surfactant material followed by a slug of thickened water, or the like.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical sectional view of a well borehole producing formation fluids from a subterranean earth formation in accordance with the teachings of the present invention;

FIG. 2 is a vertical sectional view of a plurality of well boreholes producing formation fluids from the formation of FIG. 1 in accordance with the teachings of the invention;

FIG. 3 is a graphical illustration of one feature of the present invention; and

FIGS. 4 and 5 are schematic illustrations of examples of preferred well borehole patterns for producing formation fluids from a subterranean earth formation in accordance with the teachings of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a conventional steam soak process, or other thermal soak process, the gravity layover effect, as discussed hereinabove, tends to produce a poor sweep efficiency. In a substantially non-bypassing viscous oil displacement process, the slow rate of fluid movement often results in an economically unattractive process. In the method of this invention, however, the viscous oil present in the formation is moved by a nonbypassing displacement for only a relatively short distance before it enters a steam-heated zone where the oil is thermally mobilized. The hot oil is then produced at a relatively rapid rate, during the backflow cycle of a hot fluid soak operation. Thus, although the viscous oil moves at a slow rate, since it moves across a collection zone boundary (such as the boundary of a steamheated zone) having a relatively large area, the rate of oil production from the collection zone is relatively high, due to the large volume of oil that enters the collection zone and becomes heated so that its viscosity and its resistance to flow into the well borehole are greatly reduced.

Thus, as illustrated in FIG. 1, a well borehole 10 is shown extending into communication with a hydrocarbon-bearing subterranean earth formation 11. Well borehole 10 is preferably cased by casing 12 with casing 12 cemented therein, as is well known in the art. A tubing string 13 is disposed in casing 12, preferably extending to a point substantially adjacent to the bottom of formation 11. Tubing string 13 is packed off at two locations in casing 12. Thus, tubing string 13 is packed off at packing means 14, a relatively short distance above the bottom of tubing string 13, and at packing means 15, at substantially the upper portion of formation 11, for reasons to be discussed further hereinbelow. Casing 12 is perforated, as at perforations 16, below packing means 14 so as to establish fluid communication between the bottom of tubing string 13 and the lower portion of formation 11. Casing 12 is also perforated, as at perforations 17, above packing means 15 so as to establish fluid communication between the annulus 18 formed between tubing string 13 and casing 12 and the upper portion of formation 1 1.

An annulus outlet 19 is associated with the upper end of tubing string 13 for introducing fluids into annulus 18, through perforations 17 and into the upper portion of formation 11. In like manner, fluids may be injected down tubing string 13, out perforations 16 and into the lower portion of formation 1 1.

A hot fluid soak, such as a steam-soaking operation, is preferably carried out through the openings (i.e., perforations 16) which are confined within the lower portion of formation 11, as indicated by the doubleended arrows in FIG. I. A suitable process for injecting steam into a limited zone of an oil-producing reservoir and subsequently recovering reservoir fluids through this limited injection zone to enhance the production of oil from the reservoir is described in a US. Pat. No. 3,358,762. The steam which is injected down tubing string 13 may be low quality, wet, dry, or superheated steam. The steam is preferably injected under pressure and temperature conditions that are near the minimum required for adequate rates of flow and extents of reduction in the viscosity of the oil. The steam generation can be conducted by or supplemented by the initiation of an underground combustion, for example, as described in US. Pat. 3,409,083. Such an underground combustion steam generation can be conducted at a relatively high temperature under conditions that are conducive to a thermal conversion of water-sensitive clay components to materials which are more resistant than the untreated clays to the effects of aqueous liquids that are substantially free of dissolved electrolytes.

In the embodiment shown in FIG. 1, an oil-displacing fluid is injected into annulus outlet 19 and out perforations 17 into the upper portion of formation 11. Preferably, particularly in those cases where formation 11 is relatively thick, the oil-displacing fluid is a relatively low density fluid, such as a liquid mixed with a gas and sufflcient foaming agent to form a foam having a density of less than the average density of the fluids in the pores of the reservoir. In this manner, a driving and soaking pattern is provided whereby hydrocarbons present in formation 11 in the driving zone 20.surrounding well borehole are driven or displaced into the hot soaking zone 21 from whence formation fluids are to be or are being produced back through perforations 16 and up tubing string 13 (as indicated by the double-ended arrows in FIG. 1) for example, by fluid production procedures well known in the oil production art. Hydrocarbons are recovered from the fluid being produced from hot soaking zone 21, for example by hydrocarbon recovery procedures well known in the oil production art. Thus, conventional heat exchanging, pumping, separating and heating equipment is shown associated with the upper portion of well borehole 10. A steam inlet 10a is provided, controlled by valve means 10b. An inlet 100 is provided for introducing the oil-displacing fluid into annulus outlet 19. A valve means 10d is provided between the separator and inlet 100 for controlling the injection of the fluid into well borehole 10.

Preferably, however, a pair of well boreholes 22 and 23 are extended into communication with hydrocarbon-bearing formation 11 as illustrated in FIG. 2. Well boreholes 22 and 23 are preferably cased, as by casing 24 and 25, respectively, with the casing cemented therein, as is well known in the art. Tubing strings 26 and 27 are disposed in well boreholes 22 and 23, respectively, as is also well known in the art. Packing means 28 and 29 are disposed a short distance above the bottom of tubing strings 26 and 27, which are disposed adjacent to the bottom of formation 11 in the manner discussed hereinabove with respect to the well borehole 10 of FIG. 1. Both casings 24 and 25 are perforated, as at perforations 30 and 31, respectively, so as to establish fluid communication between the lower ends of casings 24 and 25 (and thus the bottom of tubing strings 26 and 27) and the bottom portion of formation 11.

A third well borehole 32 is also extended into communication with formation 11 at a location spaced from well boreholes 22 and 23. Well borehole 32 is extended to the lower portion of formation 11 and is preferably cased, as by casing 33, with casing 33 cemented therein as is well known in the art. A tubing string 34 is disposed in well borehole 32 extending adjacent to the bottom of formation 11 and packed off from casing 23 as at packing means 35. Packing means 35 is disposed a short distance above the bottom of tubing string 34 in the manner of packing means 28 and 29 of well boreholes 22 and 23, respectively. Casing 33 is perforated below packing means 35, as at perforations 36, thus establishing fluid communication between the bottom of tubing string 35 and the lower portion of formation 1 l, as discussed hereinabove with respect to well boreholes 22 and 23.

A conventional steam soak operation is then carried out in the manner discussed hereinabove with respect to well borehole 10. Thus, steam is injected down well boreholes 22 and 23 and into formation 11, thus forming hot steam'soaking zones 37 and 38, respectively. Formation fluids are then flowed from zones 37 and 38 within formation 11 back into well boreholes 22 and 23, as is well known in the art.

The oil-displacing fluid is injected down tubing string 34 of well borehole 32 and into the lower portion of formation 11, thus forming a driving zone 39 surrounding well borehole 32. In this manner, hydrocarbons in zone 39 are driven or displaced into hot zones 37 and 38 as indicated by the arrows in FIG. 2.

Conventional heat exchanging, separating, pumping, heating, valving and inlet means are also associated with well boreholes 22, 23 and 32 for recovering hydrocarbons therefrom in the manner discussed hereinabove with respect to the well borehole 10 of FIG. 1.

In both embodiments it is preferable to heat ,the injected oil-displacing fluid to a temperature which is near that at which the steam is injected in the steamsoaking operation but is below the temperature at which the viscosity-increasing components of the oildisplacing fluid are thermally degraded. The advantages of such a step are illustrated in FIG. 3. Portions of the arrangement of FIG. 2 have been eliminated for simplification. The graph illustrates that, when the oil-displacing fluid is hot, it heats the firstcontacted oil to,a temperature at which it has a relatively low viscosity. In zones near the oil-displacing fluid in the injection well borehole 32, as for example in the cross-hatched, near-borehole portion of zone 39, the temperature tends to remain high and the resistance to flow is relatively low. As the fluids move away from well borehole 32, the temperature falls and the viscosity of the displaced oil increases as it becomes cooler. In these zones, as for example near the steam soaked zones 37 and 38, the oil temperature tends to become lower and the flow rate decreases as the injected fluid spreads over areas of larger diameters; but, as the displaced oil is moved into the steam heated zones, it is heated to a temperature at which its mobility is relatively high.

Furthermore, the region near the injection well which has been heated as a result of the heated injection fluid will result in a region of relatively low resistance to the injection of fluids. This will allow an increase in injection rate at the permissable injection pressures, which results in a shortening of the operating life of the process (lower operating costs); or in injecting the pump capacity at relatively low injection pressures, which results in lower injection costs. The use of hot water to precede the viscous drive would be advantageous in accelerating the heating of the formation near the injection well.

In both arrangements of FIGS. 1 and 2, the rate at which the oil-displacing fluid is injected into formation 11 may be varied widely and the injection may be continuous or intermittent. In general, the average rate of oil-displacing fluid injection is less than that conventionally used for waterflooding or miscible displacement. Where the viscosity of the drive-fluid approximates that of the formation fluids at reservoir temperature, the injection rate of the oil-displacing fluid is preferably that which occurs at a pressure slightly below the fracturing pressure of formation 11. For example, oil-displacing fluid injection rates of from about 100 to 1000 barrels per day have been found suitable for use in the method of the present invention. Where the injection of the oil-displacing fluid is intermittent, the inflow of the fluid is preferably sustained throughout each production cycle of the steam soaking and fluid production operation. The injected oil-displacing fluid is preferably a thickened water injected hot relative to the formation temperature. The hot injection of thickened water is advantageous in reducing the viscosity of the formation fluids adjacent to the injection point of the oil-displacing fluid.

Referring once again to the drawing, FIGS. 4 and 5 show examples of well borehole patterns which may be used in accordance with the teachings of the present invention. Thus, in FIG. 4, a five-spot pattern of well boreholes is shown, whereby four production well boreholes 40 (similar to well boreholes 22 and 23 of FIG. 2) are shown surrounding a single injection well borehole 41 (similar to well borehole 32 of FIG. 2). This five-spot pattern is repeated throughout the entire pattern of well boreholes. The inner dotted lines surrounding each production well borehole indicate the lower steam zone boundaries created utilizing the method of this invention with the outer dotted lines indicating the upper steam zone boundaries.

A line-drive" pattern of well boreholes is shown in FIG. 5. Here, the production well boreholes 42 are disposed in a linear arrangement with injection well boreholes 43 disposed adjacent thereto in a similar linear arrangement. The dotted lines surrounding the production well boreholes 42 are similar to the dotted lines of FIG. 4. The linear arrangement of well boreholes is repeated throughout the entire pattern.

The cooperating hot fluid soak and oil-displacing operations as discussed hereinabove materially enhance the sweep efficiency of the hot fluid soaks at the production well boreholes. The intervening portions of formation 11 between the injection well borehole and the production well borehole are heated and depleted, thus increasing both the rate at which hydrocarbons are produced as a result of the substantially non-bypassing viscous fluid drive and the ultimate recovery. By soaking the formation 11 at the production well boreholes, relatively vast effective production well borehole diameters are provided and the viscous fluid drive (i.e., the substantially non-bypassing hydrocarbon-displacing fluid injection) operates at a relatively high rate and efficiency clue to the reduction in pressure over the path between the injection and production well boreholes. The method of this invention as discussed hereinabove thus increases the potential profitability of a hydrocarbon-bearing formation, particularly one that is relatively thick and uniformly permeable.

What is claimed is:

1. A method of recovering relatively viscous hydrocarbons from a subterranean reservoir, comprisproviding at least one fluid conduit that extends from at least one surface location to at least two locations within the reservoir;

repetitively flowing relatively mobile hot fluid that is significantly immiscible with the reservoir hydrocarbon and is significantly hotter than the reservoir into at least one location within the reservoir and producing fluid from substantially the same location, so that a hot zone is formed within the reservoir and heated hydrocarbons are produced from the reservoir; and

flowing a hydrocarbon-displacing fluid having a mobility substantially as low as that of the adjacent hydrocarbon containing reservoir fluid into at-least one other location within the reservoir, so that reservoir hydrocarbons are displaced toward said hot zone.

2. The method of claim 1 in which said relatively mobile hot fluid is steam.

3. The method of claim 1 in which said hydrocarbondisplacing fluid is a thickened aqueous liquid.

4. The method of claim 1 in which the hydrocarbon displacing fluid is a relatively low mobility mixture of gas and liquid.

5. The method of claim 1 in which at least one of said fluid conduits extends to a location near the bottom of said reservoir.

6. The method of claim 1 in which said location into which relatively mobile hot fluid is injected and produced and said location into which hydrocarbondisplacing fluid is injected are encountered by separate wells.

7. A method for producing liquid hydrocarbons from a hydrocarbon-bearing subterranean earth formation comprising steps of: 7

extending at least one well borehole into fluid communication with substantially the bottom of said subterranean earth formation; opening a second well borehole spaced from said first well borehole into fluid communication with substantially the bottom of said earth formation;

injecting steam through said first well borehole and into said earth formation for a period of time sufficient to thermally mobilize fluids present in said earth formation, thereby forming a hot soaking zone therein; producing formation fluids from said hot soaking zone back into said first well borehole; and

injecting an essentially non-vaporous substantially non-combustible hydrocarbon-displacing fluid down said second well borehole for a period of time sufficient to drive a substantial amount of the hydrocarbons present in said earth formation outside of said hot soaking zone directly into said hot soaking zone.

8. The method of claim 7 including the step of selectively injecting steam and producing formation fluids from a plurality of well boreholes surrounding a single well borehole while injecting said hydrocarbon-displacing fluid down said single well borehole during the step of producing formation fluids from said plurality of well boreholes.

9. The method of claim 7 including the step of selectively injecting steam and producing formation fluids from a plurality of linearly disposed well boreholes displaced adjacent to a plurality of linearly disposed well boreholes while injecting said hydrocarbon-displacing fluid down said latter-mentioned well boreholes during the step of producing formation fluids from said firstmentioned well borehole.

10. A method for producing liquid hydrocarbons from a hydrocarbon-bearing subterranean earth formation comprising the steps of:

extending at least one well borehole into communication with said subterranean earth formation;

providing fluid communication between said well borehole and at least two spaced locations within said earth formation;

injecting steam through said well borehole and into one of said locations in said earth formation for a period of time sufficient to thermally mobilize fluids present in said earth formation, thereby forming a hot zone therein;

producing formation fluids from said hot zone;

hon-displacing fluid into the other of said-locations within the formation. 11. A method for producing liquid hydrocarbons from a hydrocarbon-bearing subterranean earth formation comprising the steps of:

injecting a hydrocarbon-displacing fluid into the fluids present in said earth formation, thereby forming a hot zone therein;

producing formation fluids from said hot zone;

injecting a thickened water containing viscosity-increasing components into the other of said locations within said earth formation for a period of time sufficient to displace hydrocarbons in said earth formation into said hot zone; and

heating said thickened water to a temperature about that at which said steam is injected into one of said locations in said earth formation but below the temperature at which the viscosity-increasing components of the thickened water are thermally degraded prior to injecting said thickened water into the other of said locations within said earth formation. 

2. The method of claim 1 in which said relatively mobile hot fluid is steam.
 3. The method of claim 1 in which said hydrocarbon-displacing fluid is a thickened aqueous liquid.
 4. The method of claim 1 in which the hydrocarbon-displacing fluid is a relatively low mobility mixture of gas and liquid.
 5. The method of claim 1 in which at least one of said fluid conduits extends to a location near the bottom of said reservoir.
 6. The method of claim 1 in which said location into which relatively mobile hot fluid is injected and produced and said location into which hydrocarbon-displacing fluid is injected are encountered by separate wells.
 7. A method for producing liquid hydrocarbons from a hydrocarbon-bearing subterranean earth formation comprising steps of: extending at least one well borehole into fluid communication with substantially the bottom of said subterranean earth formation; opening a second well borehole spaced from said first well borehole into fluid communication with substantially the bottom of said earth formation; injecting steam through said first well borehole and into said earth formation for a period of time sufficient to thermally mobilize fluids present in said earth formation, thereby forming a hot soaking zone therein; producing formation fluids from said hot soaking zone back into said first well borehole; and injecting an essentially non-vaporous substantially non-combustible hydrocarbon-displacing fluid down said second well borehole for a period of time sufficient to drive a substantial amount of the hydrocarbons present in said earth formation outside of said hot soaking zone directly into said hot soaking zone.
 8. The method of claim 7 including the step of selectively injecting steam and producing formation fluids from a plurality of well boreholes surrounding a single well borehole while injecting said hydrocarbon-displacing fluid down said single well borehole during the step of producing formation fluids from said plurality of well boReholes.
 9. The method of claim 7 including the step of selectively injecting steam and producing formation fluids from a plurality of linearly disposed well boreholes displaced adjacent to a plurality of linearly disposed well boreholes while injecting said hydrocarbon-displacing fluid down said latter-mentioned well boreholes during the step of producing formation fluids from said firstmentioned well borehole.
 10. A method for producing liquid hydrocarbons from a hydrocarbon-bearing subterranean earth formation comprising the steps of: extending at least one well borehole into communication with said subterranean earth formation; providing fluid communication between said well borehole and at least two spaced locations within said earth formation; injecting steam through said well borehole and into one of said locations in said earth formation for a period of time sufficient to thermally mobilize fluids present in said earth formation, thereby forming a hot zone therein; producing formation fluids from said hot zone; injecting a hydrocarbon-displacing fluid into the other of said locations within said earth formation for a period of time sufficient to displace hydrocarbons in said earth formation into said hot zone; and mixing said hydrocarbon-displacing fluid with a gas and sufficient foaming agent to form a foam having a density below about the average density of the formation fluids prior to injecting said hydrocarbon-displacing fluid into the other of said locations within the formation.
 11. A method for producing liquid hydrocarbons from a hydrocarbon-bearing subterranean earth formation comprising the steps of: extending at least one well borehole into communication with said subterranean earth formation; providing fluid communication between said well borehole and at least two spaced locations within said earth formation; injecting steam through said well borehole and into one of said locations in said earth formation for a period of time sufficient to thermally mobilize fluids present in said earth formation, thereby forming a hot zone therein; producing formation fluids from said hot zone; injecting a thickened water containing viscosity-increasing components into the other of said locations within said earth formation for a period of time sufficient to displace hydrocarbons in said earth formation into said hot zone; and heating said thickened water to a temperature about that at which said steam is injected into one of said locations in said earth formation but below the temperature at which the viscosity-increasing components of the thickened water are thermally degraded prior to injecting said thickened water into the other of said locations within said earth formation. 